Collectively, the entire adult population of the world weighs about 316 million tons, or 633 billion pounds. About 16.5 million tons of that weight is due to overweight, according to data from the United Nations and World Health Organization. That’s as much as 242 million extra people of normal weight — or roughly the entire population of Indonesia.

Most of the world’s excess weight can be found in the U.S. The average weight of an adult anywhere in the world is about 137 lbs., but the average weight of an adult in the U.S. is 178 lbss, according to the new study by researchers at the London School of Hygiene and Tropical Medicine (LSHTM). Also, consider this: while North America has only 6% of the world’s population, it accounts for 34% of the world’s weight due to obesity. In contrast, Asia has 61% of the world’s population, but 13% of its weight due to obesity.

“If every country in the world had the same level of fatness that we see in the U.S.A., in weight terms that would be like an extra billion people of world average body mass,” Professor Ian Roberts, who led the research at LSHTM, told the BBC.

Why does this matter? Because global fatness can seriously endanger the world’s food security and environmental resources. Bigger people require more energy to live: about half of all food eaten is burned up in physical activity and it takes more energy to move heavier bodies. The authors note that even at rest, a bigger body burns more energy. “When people think about environmental sustainability, they immediately focus on population. Actually, when it comes down to it, it’s not how many mouths there are to feed, it is how much flesh there is on the planet,” Roberts told the BBC.

The authors write:

Our results underscore the need to take body mass into account when considering the ecological implications of population growth. U.N. world population projections suggest that by 2050 there could be an additional 2.3 billion people. The ecological implications of rising population numbers will be exacerbated by increases in average body mass. Although the largest increase in population numbers is expected in Asia and sub-Saharan Africa, our results suggest that population increases in the U.S.A. will carry more weight than would be implied by numbers alone.

Roberts told the Telegraph that widespread initiatives that promote walking and cycling could improve public health. People may not necessarily be eating more than they were 50 years ago, he said, but many are becoming more sedentary. “We do not move our bodies so much but we are biologically programmed to eat,” he said, recommending that the world put itself on a diet to curb obesity, and make sure that the production of energy and food use fewer resources. “Unless we tackle both population and fatness our chances are slim,” he said.

The study was published [PDF] in the journal BMC Public Health to coincide with Rio+20, a U.N. conference on sustainable development during which world leaders will discuss issues of population growth and access to food and water.